Anterior Interosseous Nerve Syndrome

• Origin: Branch of median nerve 5-8 cm distal to lateral epicondyle
• Course: Passes between heads of pronator teres, under flexor digitorum superficialis arch
• Termination: Innervates FPL, radial half FDP, PQ
• No sensory component: Pure motor, explains absence of numbness

• Pinch weakness: Critical for fine motor and occupational tasks
• No sensory loss: Distinguishes from carpal tunnel or pronator syndrome
• Self-limiting: Majority resolve with observation alone
• Surgical timing: Balance spontaneous recovery against denervation atrophy

Neuritis (most common): Post-viral or idiopathic, self-limiting axonal degeneration Compression: Anatomic fibrous bands or muscle variants Trauma: Supracondylar fracture, forearm fracture, iatrogenic Inflammatory: Part of brachial plexitis (Parsonage-Turner)

FPL flexes the thumb IP joint while FDP index flexes the DIP joint. Both are required for a true tip-to-tip OK circle. When either or both are weak, the patient substitutes with a key pinch (thumb pad against radial side of index), producing the classic flat OK sign. This is pathognomonic when sensation remains intact.

• AIN palsy is uncommon but frequently under-diagnosed worldwide because of the pure motor presentation
• Neuritis predominates in all regions, often post-viral
• Compressive cases are reported more in high-resource centres with access to EMG and ultrasound
• Parsonage-Turner incidence appears similar across populations

• High-resource: Early EMG, serial ultrasound, hand therapy monitoring, microsurgical decompression available
• Limited-resource: Clinical diagnosis alone, prolonged observation, limited access to nerve surgery
• Universal principle: Outcome depends on accurate diagnosis (OK sign + PQ test) and patience with observation
• Surgery: Concentrated in specialist hand units globally, results better with shorter delay

Most authors recommend 6-12 months, but the exact point at which surgery becomes futile is not defined by high-quality data. Some units operate at 4 months while others wait 18 months for neuritis. Patient age, occupation and EMG trajectory guide individual decisions.

Ultrasound can demonstrate nerve swelling and dynamic compression but is operator-dependent. MRI better visualises muscle denervation and occult masses but is more expensive. Neither changes management in straightforward neuritis.

FDS to FPL transfer is reliable for permanent AIN loss but timing is debated. Some surgeons offer it at 12 months while others wait 18-24 months. The decision balances irreversible atrophy against the small chance of very late reinnervation.

When the AIN appears normal at exploration, some surgeons perform proximal median nerve fascicular neurolysis looking for intraneural compression. Evidence for benefit is anecdotal and the procedure risks iatrogenic injury to other fascicles.